Atomic beam device using optical pumping

ABSTRACT

A method and apparatus are disclosed for optically pumping the atoms of an atomic beam into a single trap state. An atomic beam is produced and is passed through a region of weak magnetic field to produce a magnetic splitting of energy levels. The beam is illuminated with a laser beam to produce selected excitations. The spectral distribution of the laser light is selected (1) to excite the atoms of the atomic beam to excited levels from which the atoms can decay into the trap state and (2) to avoid exciting atoms out of the trap state. Particular applications of the method and apparatus are described for atomic or ionic clocks and masers.

BACKGROUND OF THE INVENTION

This invention relates generally to atomic or ionic devices and, moreparticularly, to atomic beam devices employing optical pumping toincrease the populations of selected states. The advantages of suchpopulation enhancement are illustrated by consideration of a cesiumclock which employs an atomic beam of cesium.

In cesium, the nucleus has spin 7/2 which combines with the spin 1/2 ofthe valence electron to produce ground energy level (i.e., 6² S_(1/2))states of total angular momentum F=3 and F=4. These states have adifference in energy that can be used to produce a very accurate clocksignal. The energy of the sublevels of these hyperfine states as afunction of applied magnetic field is shown in FIG. 1. For zero appliedfield the states separate into two hyperfine states of seven F=3sublevels and nine F=4 sublevels. These states differ in energy by thehyperfine energy difference. For a strong applied magnetic field thestates separate into two groups of eight sublevels: (1) a group of F=4sublevels with m_(F) =-3, -2, . . . , +4 which increase in energy withincrease in applied field and (2) a group of sublevels with F=3 and withF=4 and m_(F) =-4 which decrease in energy with increase in appliedfield.

In the cesium tube shown in FIG. 2 the energy difference between theatomic states of cesium is used to produce a very accurate and stableclock frequency (see Hyatt, et al., "A High Performance Beam Tube forCesium Beam Frequency Standards", Hewlett-Packard Journal, September1973, p. 14-24). An oven and collimator are employed to produce a beamof cesium atoms which, because of the small energy difference betweenthe F=3 and F=4 states, are roughly evenly distributed among the sixteen6² S_(1/2) sublevels. The beam passes first through a stronginhomogeneous magnetic "A" field. It then goes through a microwavecavity in a weak homogeneous magnetic "C" field region. Next it goesthrough a second strong inhomogeneous magnetic field, the "B" field andfinally goes to the detector. The oven, "A" magnet, and "B" magnet areso arranged and fitted with beam stops that only atoms originally in theF=3, all sublevels, and the F=4 m_(F) =-4 sublevel entering the "A"magnet can get through the "B" magnet gap. The detector is placed sothat only atoms in the F=4 m_(F) =-3, -2, -1, 0, 1, 2, 3, 4 statesentering the "B" magnet will be detected. Therefore, in the absence ofRF excitation of the microwave cavity, no atoms will be detected.

The weak homogeneous magnetic "C" field is about 0.09 Gauss to produce aweak field splitting of the energies of the group of F=3 sublevels andof the group of F=4 sublevels (see FIG. 1). An rf source provides an rffield in the cavity to induce transitions from the F=3 m_(F) =0 state tothe F=4 m_(F) =0 state. The number of transitions is maximized when therf frequency equals the transition frequency corresponding to the energydifference between the 6² S_(1/2) F=3 m_(F) =0 state and the 6² S_(1/2)F=4 m_(F) =0 state. These atoms will now be deflected by the "B" magnetinto the detector which provides an amplified signal proportional to thenumber of atoms striking it per second. This signal is used to regulatea voltage controlled crystal oscillator (VCXO) to produce an outputsignal of precisely regulated frequency (in the cesium clock shown inFIG. 2 this frequency is 5 MHz). The 5 MHz signal is supplied to anoutput for use as a clock signal. The 5 MHz signal is also supplied toan rf source to produce a frequency modulated rf field of carrierfrequency equal to the transition frequency. In the cesium clock of FIG.2 the modulation frequency is 137 Hz and the carrier frequency is about9192.631774 MHz.

Because of the frequency modulation, the detector signal has a 137 Hzcomponent when the rf carrier frequency strays from the transitionfrequency. The detector signal is applied to a synchronous detector toproduce a directional error signal proportional to the 137 Hz componentof the detector signal. The error signal is applied to an integratorwhich provides an integrated error signal used to control the VCXO toproduce a 5 MHz signal.

For high gain in the feedback loop from the detector to the VCXO, theaccuracy of the 5 MHz output signal becomes that of the cesium beamtube. The fractional amount of noise introduced into the 5 MHz signal bythe cesium tube decreases with increased beam flux to the detector.Because essentially only those atoms reach the detector which leave theoven in the 6² S_(1/2) F=3 m_(F) =0 state and are excited to the 6²S_(1/2) F=4 m_(F) =0 state, fifteen-sixteenths of the beam leaving theoven is not utilized. A significant improvement in signal noise can thusbe achieved by a scheme which transfers a major fraction of the atomsleaving the oven into the 6² S_(1/2) F=3 m_(F) =0 (or F=4 m_(F) =0)state before passing through the microwave cavity.

SUMMARY OF THE INVENTION

In accordance with the illustrated preferred embodiment an oven andcollimator are employed to produce an atomic beam. The beam passesthrough a region of weak magnetic field to produce a weak fieldsplitting of the atomic energy levels. The beam is illuminated in theweak field region by a laser beam or beams having a set of frequenciesand polarizations selected to produce transitions between selectedstates. The induced transitions are selected to pump atoms into excitedstates which can spontaneously decay into the desired final state. Thefrequencies and polarizations of the laser beam are also selected toavoid inducing transitions out of the desired final state so that thefinal state is a trap into which essentially all of the atoms decay.

The disclosed invention puts essentially all the atoms in the beam intothe desired state before interaction with the rf field. For the samenumber of atoms emitted from the oven this will produce a 16-foldincrease in the detector signal compared to the prior art clock shown inFIG. 2. There is an additional increase in signal due to the widevelocity acceptance of the disclosed method. The main velocitylimitation is that atoms which do not remain in the laser beam longenough to be excited and decay a large number of times will have areduced chance of winding up in the trap state. For typical oventemperature, laser beam intensity and laser beam cross section thethermal spread in velocities will have only a small effect on thefraction of atoms reaching the trap state. In contrast, the priortechnique illustrated in FIG. 2 requires a limited spread in velocitiesbecause the deflection of atoms by the strong magnetic "A" and "B"fields is velocity dependent as well as state dependent. This increasein useful beam density improves the signal to noise ratio of thedetector signal. A further advantage of the disclosed method is that itallows elimination of the strong magnetic "A" field which can disturbthe accuracy and precision of the clock by adding a straynon-homogeneous magnetic field to the weak homogeneous magnetic "C"field. The "B" field can also be eliminated by using optical pumping fordetection, if desired.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the energy level splitting of the sixteen cesium atom 6²S_(1/2) states as a function of applied magnetic field.

FIG. 2 illustrates the structure of a typical cesium clock known in theprior art.

FIG. 3 illustrates a preferred embodiment of the disclosed invention forproducing an atomic beam having essentially all of its atoms in a singleatomic state.

FIG. 4 illustrates the atomic transitions employed in accordance withthe embodiment disclosed in FIG. 3.

FIG. 5 illustrates an alternate choice of atomic transitions employed inaccordance with the embodiment disclosed in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 3 is shown an apparatus for producing an atomic beam having allof its atoms in a single atomic state. An atomic source, such as cesium,is vaporized in an oven 31 to produce a gas. The cesium vapor passesthrough a collimator 32 to produce an atomic beam 33 of cesium atoms.Because of the elevated temperature of the oven and the small energydifference between the sixteen 6² S_(1/2) energy states, these sixteenstates are nearly equally populated.

Essentially all of the atoms in beam 33 are shifted into the 6² S_(1/2)F=4 m_(F) =0 energy state by the technique of optical pumping. (SeeGerritsen and Nienhuis, "Miltidirectional Doppler Pumping: A New Methodto Prepare an Atomic Beam Having a Large Fraction of Excited Atoms",Applied Physics Letters, Vol. 26, No. 6, Mar. 15, 1975, p. 347 for anoptical pumping application.) To accomplish this optical pumping, a d.c.current generator 34 drives current through a coil 35 to produce a weakmagnetic field 36 through which beam 33 is directed. The magnetic fieldproduces a magnetic splitting of the sixteen energy levels. The amountof splitting is shown in FIG. 1 as a function of increasing magneticfield. A laser 37 illuminates atomic beam 33 with a laser beam 38 oflight which is plane polarized in the direction parallel to magneticfield 36. Laser beam 38 has two frequencies of light selected to pumpfrom both F levels of the 6² S_(1/2) ground energy level to the F=4states of the 6² P_(1/2) excited energy level (see FIG. 4). Magneticfield 36 is selected to be weak enough that the spread in energysublevels due to the weak field splitting is less than the spectralwidth of the laser lines so that the two laser lines will inducetransitions from all sixteen 6² S_(1/2) states into the 6² P_(1/2) F=4states.

The selection rules for induced transitions due to light which is planepolarized parallel to magnetic field 36 allow only transitions obeyingΔF=-1, 0, 1 and Δm_(F) =0. However, no ΔF=0, Δm_(F) =0 transition isallowed from an m_(F) =0 state. Therefore, there will be no laserinduced transitions out of the 6² S_(1/2) F=4 m_(F) =0 state.Spontaneous emissions, however, are allowed for Δm_(F) =-1, 0, 1 so thatatoms can decay from the 6² P_(1/2) F=4 states into the 6² S_(1/2) F=4m_(F) =0 state. This state therefore acts as a trap into whichessentially all of the atoms in beam 33 fall. Perturbations, such asatomic collisions and stray fields can produce unwanted transitions outof the trap state. Magnetic field 36 is therefore selected to be largeenough that the energy separation between the trap state and adjacentstates is large enough that the characteristic time of such a transitionis much longer than the transit time of the atomic beam through thelaser beam.

The m_(F) =0 state is a particularly useful trap state because timestandards such as a cesium clock typically use a transition with Δm_(F)=0 between m_(F) =0 states. The reason for this is that this transitionfrequency has the smallest magnetic field dependence. For weak magneticfields the frequency f corresponding to a transition varies as f=f_(o)+am_(F) B+bB². In a cesium clock, typical values of a and b are a≈7×10⁵Hertz/Gauss and b≈427 Hertz/(Gauss)². The energies of the m_(F) =0states are thus less sensitive to variation of the magnetic "C" field inthe cesium tube. Therefore, the beam is selected to enter in the F=4m_(F) =0 state and the polarization and frequency of the rf field isselected to induce transitions to the F=3 m_(F) =0 state.

The choice of laser 37 is restricted by only a few requirements. Thelaser must be tunable to and produce the two frequencies selected foruse in the optical pumping scheme or two lasers may be used. Also, thelaser line width must be less than the hyperfine splitting between theenergy levels into which the atoms are excited. If this latterrequirement were violated in the example discussed above, the laser linewhich excites atoms out of the 6² S_(1/2) F=4 level would excite atomsto the 6² P_(1/2) F=3 level as well as the 6² P_(1/2) F=4 level. Thislaser would therefore excite atoms out of the 6² S_(1/2) F=4 m_(F) =0state so that this state would not serve as a trap. For increased powerefficiency, the laser should be plane polarizeable, but this is notessential. Alternatively, at a loss of up to half of the laser output,the plane polarized laser beam can be produced by passing an unpolarizedor circularly polarized beam through a conventional polarizer.

The laser should produce a laser beam having a cross section which iswide enough that the entire atomic beam passes through the laser beam.The height of the cross section must be large enough that the atoms inthe atomic beam remain within the laser beam for many spontaneous decayhalf-lifes to enable essentially all of the atoms to reach the trapstate. However, because the typical velocity of an atomic beam is on theorder of 10⁴ cm/sec and a spontaneous decay half-life is on the order of10 nsec, this requirement should not limit the choice of laser. Thelaser should also be powerful enough that the average time required toexcite a laser illuminated ground state atom is much shorter than thetime required for the atom to pass through the laser beam. This level ofintensity is required to enable enough transitions that essentially allof the atoms will reach the trap state. A laser having at least a fewmilliwatts power rating should be adequate. In the example discussedabove, the laser was selected to be a 50 milliwatt continuous wave dyelaser using HITC dye pumped with krypton. In an alternate embodiment inwhich the cesium atoms are excited into the F=4 states of the 7² P1/2excited energy level, the required light of approximately 4555 Awavelength can be obtained using Cumarin dye pumped with krypton.

The two laser lines can be obtained by modulating the laser to producesidebands. For a laser of frequency f_(c) and modulation frequencyf_(m), the spectral distribution of the laser light has components atf_(c) -f_(m), f_(c) and f_(c) +f_(m). The modulation of the laser lightcan be amplitude modulation, phase modulation, or a combination ofamplitude and phase modulation. The required pair of laser lines cantherefore be provided by using any two of these three lines andselecting f_(c) and f_(m) to produce the frequencies required for thetransition scheme of FIG. 4. The pair of laser frequencies canalternatively be obtained by employing two lasers, each providing one ofthe selected frequencies.

The disclosed method is not limited to use only in cesium clocks.Clearly, the same techniques apply to any atomic or ionic clock ordevice requiring atoms or ions in a single ground level state. Indeedthe method applies to any composite particle (i.e., a bound state ofmore than one particle; for example an atom is a bound state of protons,neutrons and electrons) having energy states between which transitionscan be induced by optical pumping. Thus, the relevant atoms or ions willsometimes be referred to in the present specification and claims as"particles". For example, the method also can be applied to masers. Inmasers, a beam of atoms in an excited state is passed through amicrowave cavity to amplify the signal in the cavity via stimulatedemission of radiation or to provide stable oscillation. This techniqueof optical pumping can also be applied in non-beam type atomic devicesto pump atoms or ions into a single ground state.

The method is also more general than the single example discussed abovemight indicate. The primary criteria in selecting polarizations andspectral distribution of the laser light are: (1) low energy atoms areto be excited to levels from which the atoms can decay into the trapstate and (2) there are to be no laser induced transitions out of thetrap state. For example, in FIG. 4, the atoms in the 6² S_(1/2) F=3level could alternatively be excited to the 6² P_(1/2) F=3 level sincethe states in that level can decay to the 6² S_(1/2) F=4 level byspontaneous emission. However, to avoid exciting atoms out of the 6²S_(1/2) F=4 m_(F) =0 trap state, the excitations from the 6² S_(1/2) F=4level must be only to levels having F=4 and the light must be planepolarized along the direction of the magnetic field. Unfortunately, thisalternate embodiment also produces a 6² S_(1/2) F=3 m_(F) =0 trap state.

When the method is used in conjunction with an atomic clock such as thecesium clock of FIG. 2, the magnetic "A" field is eliminated andreplaced by the combination of the weak magnetic field and laser. Themagnetic field is oriented parallel to the "C" field of the cesium beamtube so that the state into which the atoms are pumped is an m_(F) =0state relative to the direction of the "C" field. In addition, thedetector is shifted to intercept the part of the beam deflected towardthe strong field region of the "B" field. Thus, in contrast to thecesium clock of FIG. 2, the cesium clock employing the optical pumpingscheme operates to direct cesium atoms in the 6² S_(1/2) F=4 m_(F) =0state into the cesium beam tube cavity and to induce these atoms intothe 6² S_(1/2) F=3 m_(F) =0 state.

An F=3 m_(F) =0 trap state can be produced for use in a clock as in FIG.2 by employing laser light which is plane polarized parallel to the weakmagnetic field and has lines at the two frequencies suitable forproducing the excitations shown in FIG. 5. However, this embodiment hasthe disadvantage of producing additional trap states in the 6² S1/2 F=4m_(F) =±4 states. However, these states are substantially less populatedthan the 6² S_(1/2) F=3 m_(F) =0 state so that the use of thisembodiment in a cesium clock still provides significant improvement ofthe cesium beam tube signal to noise ratio. The "B" field and detectorcan be replaced with an optical pumping detection scheme if desired.

I claim:
 1. A method of producing source particles in a single state,said particles being selected from the group consisting of atoms, andions, and said method comprising the steps of:(a) selecting opticalpumping frequencies and polarizations (i) to avoid pumping saidparticles out of the trap state and (ii) to pump particles in all otherground level states to at least one excited energy level from which theparticles can decay into the trap state; and (b) illuminating the sourceparticles with light having the selected optical pumping frequencies. 2.A method as recited in claim 1 further comprising the steps of:(c)forming the particles into a particle beam; and (d) directing theparticle beam through a pumping region in which the particles areilluminated with light having the selected optical pumping frequencies.3. A method as recited in claim 1 further comprising the step ofproviding a magnetic field in a pumping region which contains the sourceparticles while they are being illuminated with said light.
 4. A methodas recited in claim 3 wherein the frequencies are selected to inducetransitions from the hyperfine energy level containing the trap stateonly to other hyperfine energy levels of equal total angular momentum Fand the polarization is selected to be planar with a polarization axisoriented parallel to the magnetic field.
 5. An improved atomic clock ofthe type in which an atomic source provides atoms to which an rf fieldis applied to induce transitions from an initial state to a final state,in which the frequency of the rf field is regulated in response to aclock signal from a variable frequency oscillator, in which the clocksignal is also provided at an output for timing purposes, and in whichthe rate of transitions is detected to hold the clock signal at afrequency which maximizes the rate of transitions, wherein theimprovement comprises means for optically pumping the atoms into atleast one excited level from which the particles can decay into a singleground level trap state before the rf field is applied to the atoms,said trap state serving as the initial atomic state in the atomic clock.6. An improved atomic clock as recited in claim 5 wherein the means foroptically pumping comprisesa light source to illuminate the atoms withlight having frequencies and polarizations which excite all atoms inground energy level states, other than those atoms in the trap state, toan excited energy level.
 7. An improved atomic clock as recited in claim6 wherein the light source comprises a plane polarizeable laser.
 8. Animproved atomic clock as recited in claim 6 further comprising means forproviding a magnetic field in a pumping region which contains the atomswhile they are being illuminated with said light.
 9. An improved ionicclock of the type in which an ionic source provides ions to which an rffield is applied to induce transitions from an initial state to a finalstate, in which the frequency of the rf field is regulated in responseto a clock signal from a variable frequency oscillator, in which theclock signal is also provided at an output for timing purposes, and inwhich the rate of transitions is detected to hold the clock signal at afrequency which maximizes the rate of transitions, wherein theimprovement comprises means for optically pumping the ions into a singleground level trap state before the rf field is applied to the ions, saidtrap state serving as the initial ionic state in the ionic clock.
 10. Animproved ionic clock as recited in claim 9 wherein the means foroptically pumping comprises:means for providing a magnetic field in apumping region containing the ions; and a light source to illuminate theions in the pumping region with light having frequencies andpolarizations which excite all ions in ground energy level states, otherthan those ions in the trap state, to an excited energy level.
 11. Animproved ionic clock as recited in claim 10 wherein the light sourcecomprises a plane polarizeable laser.
 12. An improved maser of the typein which atoms in an upper hyperfine sublevel of the atomic groundenergy level are provided to a microwave cavity to stimulate emission ofmicrowave frequency electromagnetic radiation from the atoms, theimprovement comprising optically pumping the atoms into a trap state insaid hyperfine sublevel.